Evolution of the sludge mineral composition enhances operation performance of the aerobic granular sludge reactor coupled with iron electrolysis

Abstract

Herein, the effects and mechanisms of the element Fe as mineral forms present in the aerobic granular sludge (AGS) matrix on the reactor performance were systematically investigated in an AGS reactor coupled with iron electrolysis (denoted as Re). As comparison, a control AGS reactor (denoted as Rc) was operated in parallel without an iron electrolysis unit. Results showed that the mineral-rich AGS was rapidly formed in Re, whose specific growth rate of granular diameter reached 0.13 day−1. During the granulation process, the dominating minerals present in Re sludge gradually evolved from amorphous iron oxides to crystalline iron phosphates. In contrast, the formed AGS in Rc was slow-growing and mineral-deficient. Accordingly, the roles of evolution of sludge mineral composition on accelerating granulation process were analyzed and verified. Moreover, Re exhibited higher total nitrogen (TN) and total phosphorus (TP) removal efficiencies, respectively 1.18 and 1.97 fold higher than those of Rc. Mechanisms investigation indicated that the enrichment of the genus Dechloromonas (with relative abundance of 5.45%) in Re sludge could utilize the electrolytic Fe(II) to reduce nitrate for TN removal, and the formation of other iron species contributed to TP removal through initiating various physicochemical reactions with phosphate, including coprecipitation with Fe(III) and adsorption by iron oxides. Overall, this study provides theory guidance and control views for practical applications of Fe-related strategies aiming to enhance operation performance of AGS reactors.